1. Field of the Invention
The present invention relates to a signal processing method and a displacement detection apparatus which uses the same, and more particularly to a displacement detection apparatus such as a rotary encoder or a linear encoder which detects displacement information of a moving object from a displacement signal relating to the moving object obtained from detection means.
2. Related Background Art
In a prior art rotary encoder or linear encoder or the like, two sinusoidal waves of different phases (2-phase sinusoidal wave signals or namely, a sine wave and a cosine wave) relating to displacement information are generated by detection means, and a displacement and a direction of movement (direction of displacement) are detected based on the two sinusoidal wave signals.
In a prior art encoder, a plurality of divisional pulses having a phase difference corresponding to a division unit are made from the sine wave signal and the cosine wave signal to enhance a detection resolution of the displacement information relating to the moving object.
FIG. 1 shows a block diagram of a main portion for illustrating a signal processing method for the two sinusoidal wave signals in the prior art encoder. Signal waveforms in the circuit of FIG. 1 are shown by solid lines in FIG. 2.
In FIG. 1, numerals 51 and 52 denote input terminals to which a sine wave signal Cl(a) and a cosine wave signal S1(b) are respectively inputted. Namely, a signal b (cosine wave) having a 90 degrees phase difference to a signal a (sine wave) applied to the input terminal 51 is inputted from the input terminal 52. An inverting circuit 53 gets a signal c which has a 180 degrees phase difference to the signal a. Those three signals are appropriately weight-summed through resistors to insert a sinusoidal wave signal of a desired angle.
In FIG. 1, all resistors R are of equal value to get signals d and e having phase differences of 45 degrees and 135 degrees, respectively. Those signals are converted to square waves by comparators 54, 55, 56 and 57 to produce 2-phase square wave signals having a 90 degrees phase difference by a pulse circuit 58. In this manner, a signal of a higher resolution is produced from the sine wave signal and the cosine wave signal obtained from photo-sensing means.
Japanese Patent Application Laid-Open No. 3-68812 discloses an insertion method of a position measurement signal which attains a high resolution by outputting to insert data in a memory by using an A/D-converted version of 2-phase sinusoidal wave signals as an address.
In a prior art displacement detection apparatus such as an encoder or a laser interferometer, any desired resolution can be attained by weighted resistors.
However, the following problems arises when the resolution is to be enhanced.
(a) A precision of the summing resistors is required and a proper value may not be sometimes attained within a normal range. PA0 (b) An insertion precision is lowered by an error of the summing resistors, an input offset of a comparator and an influence by a variation of a propagation delay time. A broken line in FIG. 2 shows a signal timing when the signal C1 includes an offset component. In the past, the offset component is adjusted by a potentiometer or the like in an electric circuit. PA0 (c) The number of comparators increases whereby a circuit scale is large. For example, when 2-phase square wave signals having a frequency which is 64 times as high as that of the original signal is to be got, 32 comparators must be required.